1. Field
The following description relates to a direct current (DC) power cable with excellent space charge reducing effect.
2. Description of Related Art
A power cable being currently used in the country includes a conductor 1, an inner semiconductive layer 2, an insulation 3, an outer semiconductive layer 4, a lead sheath 5, and a polyethylene (PE) sheath 6, as shown in FIGS. 1A and 1B.
Crosslinked polyethylene (XLPE) has been widely used as the insulation 3 of the power cable. However, because XLPE is difficult to recycle, increasingly strict restrictions for global environmental protection may prevent the use of XLPE. Also, when premature crosslinking or scorch occurs to XLPE, a long-term extrusion performance may disadvantageously reduce, resulting in ununiform production capability. Furthermore, when XLPE is subject to a crosslinking process using a crosslinking agent, crosslinking by-products such as alpha-methylstyrene or acetophenone may be generated. To remove the crosslinking by-products, a degassing process should be added, and as a result, the process time and cost increase.
Moreover, in case where a power cable with an insulation of XLPE is used as a high voltage transmission line, problems may occur. The worst problem is that when a high voltage DC is applied to the cable, a space charge is liable to generate due to movement of electric charges from an electrode into the insulation and the influence of crosslinking by-products. And, if such a space charge is accumulated in the insulation by a high voltage DC applied to the power cable, the electric field strength near a conductor of the power cable increases, and the breakdown voltage of the cable reduces.
To solve the problem, solutions have been suggested to form an insulation using magnesium oxide. Magnesium oxide basically has a face centered cubic (FCC) crystal structure, but may have various shapes, purity, crystallinity and properties depending on synthesis methods. The shape of magnesium oxide includes cubic, terrace, rod-like, porous and spherical shapes, as shown in FIGS. 2A to 2E, and each shape may be used depending on specific properties. In particular, spherical magnesium oxide is used to suppress a space charge of a power cable, as suggested in Japanese Patent Nos. 2541034 and 3430875. As mentioned above, studies have been steadily made to suppress a space charge in a power cable with an insulation.
However, in the conventional DC power cable, a large amount of carbon black is contained in a conductive composition used to form the inner semiconductive layer 2 or the outer semiconductive layer 4, relative to a base resin. A resulting DC power cable has an increase in volume and weight, and a reduction in dispersion of carbon black in the base resin. Therefore, there is a need for studies on materials usable as conductive particles in place of carbon black.